Adaptive receiving arrays are very useful in many wireless communication applications, including radar, wireless networks, RFID etc. Adaptive receiving arrays can converge energy, receive signals from a desired direction and reject interference from unwanted directions. This increases the communication range and allows spatial multiplexing. Beam-forming techniques using phase shifters can be used to implement an adaptive receiving array. However, the use of these techniques is generally limited due to the high cost, complexity and high insertion loss of microwave phase shifters.
Low cost phase shifters using diode loaded transmission lines or micro-electromechanical systems (MEMS) have been proposed. However, in these techniques, the signal energy has to be split between the phase shifting elements by a power divider which has high insertion loss. Therefore the energy loss increases with each additional antenna as the signal needs to be split more ways.
Various active phase shift control circuits have been proposed including circuits using a digital signal processor or direct digital synthesizer. However, those circuits are very expensive.
The inventors previously invented an indirect controlled phased source (ICPS), which was disclosed in their application U.S. Ser. No. 11/739,198 filed on 24 Apr. 2007, which is incorporated herein by reference. An example of an ICPS is shown in FIG. 1. A VCO 450 outputs a radio frequency signal 460, which is phase locked to a low frequency reference signal 410 by a phased locked loop 430. A variable phase shifter 420 is provided between the low frequency reference oscillator 410 and the phase locked loop 430. The variable phase shifter 420 shifts the phase of the low frequency reference signal which is input to the phase locked loop. In this way the output phase of the VCO can be indirectly controlled by adjusting the phase shift provided by the phase shifter 420.